A new government report issued this week assures fliers that full-body X-ray scanners in U.S. airports are safe. According to the report by the Department of Homeland Security, the machines use so low a dose of radiation that passengers would have to walk through a scanner 47 times a day for a full year to exceed yearly limits of radiation exposure.

The findings are based on the agency’s interviews with scientists and a review of research on the machines, the report said.

About 250 of the scanners, known as backscatter machines, are currently operating in 39 U.S. airports. (These are distinct from the other type of full-body scanner used in U.S. airports, which employ millimeter-wave technology that isn’t thought to pose a health risk.) Backscatter X-ray scanners use an “extremely low dose of ionizing radiation,” according to the report, to capture a full-body image of passengers in order to detect threats like weapons and explosives that may be missed by traditional metal detectors. Transportation officials favor full-body units, noting that metal detectors failed to find the plastic explosives carried by the so-called underwear bomber, Umar Farouk Abdulmutallab, who attempted to take down a plane in 2009.

But the scanners continue to make headlines, attracting criticism from public officials and wary travelers. The European Union banned the use of the X-ray machines in November due to health concerns.

According to the government report, with each scan, the average passenger receives a dose of about 1.46 microrems, or millionths of a rem, the standard measure of a person’s radiation exposure. In comparison, a dental X-ray delivers up to 15 millirems, or thousandaths of a rem, and a mammogram delivers a dose of about 70 millirems. The report suggests that the radiation exposure from a single airport scan is equivalent to the exposure from the soil and air during 1.5 hours at sea level.

The report also provides recommendations for proper scanner use, advising the Transportation Security Administration (TSA) to ensure that the machines are consistently calibrated, check for unintended radiation emissions, and make sure that staff members operating the machines are well trained.

“We believe this report fully endorses TSA’s extensive efforts to keep the traveling public safe, which is our agency’s ultimate priority,” said TSA Administrator John Pistole in a statement in response to the report.

However, some government officials maintain their doubts. Senator Susan Collins, who called for an independent study of the health effects of the scanners during a November Homeland Security Committee hearing, is unsatisfied by the new report. Pistole originally agreed to an independent study, but later said the report would be a sufficient substitute.

“This report is not the report I requested,” Collins said in a statement. “An independent study is needed to protect the public and to determine what technology is worthy of taxpayer dollars. Among other things, such a study should examine the risk of backscatter X-ray technology from a public health perspective in addition to the potential risk to individuals and TSA employees.”

Collins said that further study was needed particularly for frequent fliers and other vulnerable people, including pregnant women and airport workers. She also said that signs should be posted at airports clearly informing passengers of their right to forgo X-ray screening and opt for a physical pat down. “Surely passengers should also be well informed of their screening options,” said Collins.